cmd/go: fail go clean command when failed to find go cache directory

src/cmd/go/go_test.go

@@ -197,7 +197,7 @@ func TestMain(m *testing.M) {
 		defer removeAll(testTmpDir)
 	}
 
-	testGOCACHE, _ = cache.DefaultDir()
+	testGOCACHE, _, _ = cache.DefaultDir()
 	if testenv.HasGoBuild() {
 		testBin = filepath.Join(testTmpDir, "testbin")
 		if err := os.Mkdir(testBin, 0777); err != nil {

src/cmd/go/internal/cache/default.go

@@ -34,11 +34,11 @@ See golang.org to learn more about Go.
 // initDefaultCache does the work of finding the default cache
 // the first time Default is called.
 func initDefaultCache() Cache {
-	dir, _ := DefaultDir()
+	dir, _, err := DefaultDir()
+	if err != nil {
+		base.Fatalf("build cache is required, but could not be located: %v", err)
+	}
 	if dir == "off" {
-		if defaultDirErr != nil {
-			base.Fatalf("build cache is required, but could not be located: %v", defaultDirErr)
-		}
 		base.Fatalf("build cache is disabled by GOCACHE=off, but required as of Go 1.12")
 	}
 	if err := os.MkdirAll(dir, 0o777); err != nil {
@@ -71,7 +71,7 @@ var (
 // DefaultDir returns the effective GOCACHE setting.
 // It returns "off" if the cache is disabled,
 // and reports whether the effective value differs from GOCACHE.
-func DefaultDir() (string, bool) {
+func DefaultDir() (string, bool, error) {
 	// Save the result of the first call to DefaultDir for later use in
 	// initDefaultCache. cmd/go/main.go explicitly sets GOCACHE so that
 	// subprocesses will inherit it, but that means initDefaultCache can't
@@ -100,5 +100,5 @@ func DefaultDir() (string, bool) {
 		defaultDir = filepath.Join(dir, "go-build")
 	})
 
-	return defaultDir, defaultDirChanged
+	return defaultDir, defaultDirChanged, defaultDirErr
 }

src/cmd/go/internal/clean/clean.go

@@ -153,7 +153,10 @@ func runClean(ctx context.Context, cmd *base.Command, args []string) {
 	sh := work.NewShell("", &load.TextPrinter{Writer: os.Stdout})
 
 	if cleanCache {
-		dir, _ := cache.DefaultDir()
+		dir, _, err := cache.DefaultDir()
+		if err != nil {
+			base.Fatal(err)
+		}
 		if dir != "off" {
 			// Remove the cache subdirectories but not the top cache directory.
 			// The top cache directory may have been created with special permissions
@@ -180,7 +183,10 @@ func runClean(ctx context.Context, cmd *base.Command, args []string) {
 		// Instead of walking through the entire cache looking for test results,
 		// we write a file to the cache indicating that all test results from before
 		// right now are to be ignored.
-		dir, _ := cache.DefaultDir()
+		dir, _, err := cache.DefaultDir()
+		if err != nil {
+			base.Fatal(err)
+		}
 		if dir != "off" {
 			f, err := lockedfile.Edit(filepath.Join(dir, "testexpire.txt"))
 			if err == nil {

src/cmd/go/internal/envcmd/env.go

@@ -131,7 +131,7 @@ func MkEnv() []cfg.EnvVar {
 				env[i].Changed = true
 			}
 		case "GOCACHE":
-			env[i].Value, env[i].Changed = cache.DefaultDir()
+			env[i].Value, env[i].Changed, _ = cache.DefaultDir()
 		case "GOTOOLCHAIN":
 			env[i].Value, env[i].Changed = cfg.EnvOrAndChanged("GOTOOLCHAIN", "")
 		case "GODEBUG":

src/cmd/go/internal/help/helpdoc.go

@@ -505,7 +505,7 @@ General-purpose environment variables:
 		The directory where 'go install' will install a command.
 	GOCACHE
 		The directory where the go command will store cached
-		information for reuse in future builds.
+		information for reuse in future builds. Must be an absolute path.
 	GOCACHEPROG
 		A command (with optional space-separated flags) that implements an
 		external go command build cache.

src/cmd/go/internal/modindex/read.go

@@ -151,7 +151,7 @@ func GetPackage(modroot, pkgdir string) (*IndexPackage, error) {
 // using the index, for instance because the index is disabled, or the package
 // is not in a module.
 func GetModule(modroot string) (*Module, error) {
-	dir, _ := cache.DefaultDir()
+	dir, _, _ := cache.DefaultDir()
 	if !enabled || dir == "off" {
 		return nil, errDisabled
 	}

src/cmd/go/internal/test/test.go

@@ -839,7 +839,7 @@ func runTest(ctx context.Context, cmd *base.Command, args []string) {
 	// Read testcache expiration time, if present.
 	// (We implement go clean -testcache by writing an expiration date
 	// instead of searching out and deleting test result cache entries.)
-	if dir, _ := cache.DefaultDir(); dir != "off" {
+	if dir, _, _ := cache.DefaultDir(); dir != "off" {
 		if data, _ := lockedfile.Read(filepath.Join(dir, "testexpire.txt")); len(data) > 0 && data[len(data)-1] == '\n' {
 			if t, err := strconv.ParseInt(string(data[:len(data)-1]), 10, 64); err == nil {
 				testCacheExpire = time.Unix(0, t)

src/cmd/go/internal/work/shell.go

@@ -127,7 +127,7 @@ func (sh *Shell) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) e
 	// Otherwise fall back to standard copy.
 
 	// If the source is in the build cache, we need to copy it.
-	dir, _ := cache.DefaultDir()
+	dir, _, _ := cache.DefaultDir()
 	if strings.HasPrefix(src, dir) {
 		return sh.CopyFile(dst, src, perm, force)
 	}

src/cmd/go/testdata/script/clean_cache_n.txt

@@ -18,6 +18,11 @@ go clean -cache
 ! go clean -cache .
 stderr 'go: clean -cache cannot be used with package arguments'
 
+# GOCACHE must be an absolute path.
+env GOCACHE=.
+! go clean -cache
+stderr 'go: GOCACHE is not an absolute path'
+
 -- main.go --
 package main